25 March 2021 Measuring the topology of a polaritonic analog of graphene

Scanning electron microscopy image of a honeycomb lattice of coupled micropillars

A recent study published in PRL reports the measurement of topological invariants in artificial graphene. Topological insulators attracted much interest in the last decades. These exotic materials are characterized by a global topological invariant, an integer, that cannot be deformed by local perturbations such as disorder or interactions. This robustness makes them ideal candidates for applications in metrology or quantum computation. For example, materials displaying the integer quantum Hall effect yield extremely robust plateaux in their transverse conductivity. Such robustness comes from the celebrated bulk-edge correspondence, which dictates that the topological invariant of the bulk is equal to the number of protected conducting edge states.

A distinct situation arises in 2D crystals like graphene, possessing both time reversal and chiral symmetry. Such systems do not have a gap. Nevertheless, they can present edge states which are robust against perturbations respecting the symmetry of the system. These states can be linked to a topological invariant defined over reduced one-dimensional subspaces of their Brillouin zone.

In a recent article published in Physical Review Letters and highlighted as an Editor’s Suggestion, researchers of the Université Paris-Sud and Université de Lille, in collaboration with ICFO researcher Alexandre Dauphin and Pietro Massignan from the Universitat Politècnica de Catalunya (UPC), report the measurement of such topological invariants in artificial graphene.

The researchers demonstrated a novel scheme based on a hybrid position- and momentum-space measurement to directly access these 1D topological invariants in lattices of semiconductor microcavities confining exciton-polaritons. They showed that such technique can be applied both to normal and strained graphene.

This work opens the door to a systematic study of such systems in the presence of disorder or interactions.